Synthesis, Structure, Luminescence, and Magnetic Properties of a Single-Ion Magnet “mer”‑[Tris(N‑[(imidazol-4-yl)-methylidene]-dl-phenylalaninato)terbium(III) and Related “fac”-dl-Alaninato Derivative

Two TbIII complexes with the same N6O3 donor atoms but different coordination geometries, “fac”-[TbIII(HLdl‑ala)3]·7H2O (1) and “mer”-[TbIII(HLdl‑phe)3]·7H2O (2), were synthesized, where H2Ldl‑ala and H2Ldl‑phe are N-[(imidazol-4-yl)­methylidene]-dl-alanine and -dl-phenylalanine, respectively. Each TbIII ion is coordinated by three electronically mononegative NNO tridentate ligands to form a coordination geometry of a tricapped trigonal prism. Compound 1 consists of enantiomers “fac”-[TbIII(HLd‑ala)3] and “fac”-[TbIII(HLl‑ala)3], while 2 consists of “mer”-[TbIII(HLd‑phe)2(HLl‑phe)] and “mer”-[TbIII(HLd‑phe)­(HLl‑phe)2]. Magnetic data were analyzed by a spin Hamiltonian including the crystal field effect on the TbIII ion (4f8, J = 6, S = 3, L = 3, gJ = 3/2, 7F6). The Stark splitting of the ground state 7F6 was evaluated from magnetic analysis, and the energy diagram pattern indicated easy-plane and easy-axis (Ising type) magnetic anisotropies for 1 and 2, respectively. Highly efficient luminescences with Φ = 0.50 and 0.61 for 1 and 2, respectively, were observed, and the luminescence fine structure due to the 5D4 → 7F6 transition is in good accordance with the energy diagram determined from magnetic analysis. The energy diagram of 1 shows an approximate single-well potential curve, whereas that of 2 shows a double- or quadruple-well potential within the 7F6 multiplets. Complex 2 displayed an onset of the out-of-phase signal in alternating current (ac) susceptibility at a direct current bias field of 1000 Oe on cooling down to 1.9 K. A slight frequency dependence was recorded around 2 K. On the other hand, 1 did not show any meaningful out-of-phase ac susceptibility. Pulsed-field magnetizations of 1 and 2 were measured below 1.6 K, and only 2 exhibited magnetic hysteresis. This finding agrees well with the energy diagram pattern from crystal field calculation on 1 and 2. DFT calculation allowed us to estimate the negative charge distribution around the TbIII ion, giving a rationale to the different magnetic anisotropies of 1 and 2.